If we are entering the park at 9am when it opens with a one day pass can we leave to go have breakfast then come back to the.park? How long is that journey?

You will have to go thru security and get magic bands scanned to get in to park 30 minutes at least to do anything and turn around and walk to Chef Mickeys. With kids that means unless your breakfast is after 10:30 you are wasting your time.

You should be able to chef mickey is at the contempary resort about a 5 to 10 miniutes walk or you could catch a resort monorail which will be longer due to waiting.

Yep..you can walk to and from contemporary. I would recommend parking at contemporary resort. Its a 9 min walk.

Last time we went there, we had a reservation at 7:30 am, so, what I recommend is to go and have breakfast first, then from the contemporary resort is just a 10 min walk to magic kingdom.